Polarization mode dispersion (PMD) is an effect in fiber-optic systems which causes the transmission signals to spread in time, thereby causing signal degradation by intersymbol interference. The effect is essentially due to deviations from the ideal circular symmetry of the fiber, which may be caused by production imperfections or external influences. PMD may also be present in discreet optical components used in optical amplifiers. It is important to note that the state of polarization, as well as the instantaneous value of the PMD, can change with time. These changes may be very slow (e.g., if caused by changes of the fiber temperature) or fast (e.g., if caused by mechanical vibrations). PMD is a limiting factor for long-distance, high bit rate systems, such as long-haul wavelength division multiplexing (WDM) systems operating at bit rates of 10 Gb/s and above.
The PMD tolerance of a transmission system depends on its design parameters such as data rate, modulation format, forward error correction (FEC) and the available system margin. Also, PMD can be compensated for by the use of an optical PMD compensator (PMDC), which is placed in front of the receiver.
There are a number of problems associated with optical PMDCs. One of these is limited tracking speed (i.e., inefficient ability to follow a fast change in the polarization orientation and the PMD of the signal). The optical PMDC compensates PMD by adding to the signal the same amount of PMD but with the opposite sign as the transmission link resulting, ideally, in a zero PMD. A loss of tracking is likely to increase the PMD at the receiver over the value of PMD created on the transmission link.
If the optical receiver experiences high PMD, the bit error rate (BER) may increase beyond correcting capabilities of the forward error correction algorithms. Such events will therefore lead to bit errors which are seen by the user of the system. Because such errors often occur in bursts, they are difficult to monitor.